Added persistent underground sites and clickable passages

I added underground exploration through four kinds of surface entrance. Each seeded site has one to eight floors, with its own encounters, stairs and saved floor state. Returning to an explored floor preserves its progress instead of starting the site again.

A passage now takes one click: the survivor approaches the entrance and uses it when close enough. Moving elsewhere cancels the approach, and a transition guard prevents repeated entry. Upward stairs have their own pickable geometry for the return route.

“Four site types lead to saved underground floors, with one-click approach, stairs and a return to the surface.”

The research entrance now has a native service-lift vestibule and a forecourt fitted to the existing ground. The wider environment rebuild is still underway; this update covers the working exploration and passage controls, not finished art for every interior.